1. Field of the Invention
This invention relates in general to electronic circuitry and specifically to transmission gates for electronic circuitry.
2. Description of the Related Art
Transmission gates are used to selectively enable or disable a current path in electronic circuitry. In one example, a transmission gate utilizes an NMOS transistor and a PMOS transistor in parallel with the gates of the transistors being connected to signals inverted from each other to selectively enable and disable a current path through the transistors.
With such an example, the transistors have to be sized to handle the voltage drop between the drain and source of each transistor when the transmission gate is in a non conductive state. Thus, the voltage drop across a transmission gate is limited by the “safe operating voltage area” of a transistor of the transmission gate.
The safe operating voltage area of a transistor can be defined as the voltage conditions over which the transistor can be expected to operate without self-damage. The safe operating voltage area of a MOSFET is determined by a number of factors that cause damage to a transistor due to an over voltage condition. For example, gate oxide damage can occur if the gate to source or gate to drain voltage exceeds the breakdown voltage of the gate oxide. Junction breakdown can occur if the source to body or drain to body reverse junction bias voltage exceeds the breakdown voltage of the junction. A turn on of the bipolar junction transistor parasitic of a MOSFET can occur when the drain to source voltage exceeds a certain voltage limit. In addition to catastrophic failure, damage to a transistor due to operating outside the safe operating voltage area can also include degradation of device performance or operating life of the transistor due to, for example, the effects of charge trapping in the gate oxide, hot carrier degradation, or negative-bias temperature instability. Other factors may contribute to defining the voltage levels of the safe operating voltage area of a transistor.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The Figures are not necessarily drawn to scale.